1. Field of the Invention
The present invention relates to a solid state image pickup device that optically reads an image and converts the image into digital data.
2. Related Background Art
Up to now, a large number of image reading apparatuses have been developed. Even in medical fields, in order to conduct electronic filing, remote diagnosis, a diagnostic support using a computer, and the like, there have been developed apparatuses that take a medical image, in particular, an X-ray film image and convert the medical image into digital data.
According to such image reading apparatuses, an X-ray film is irradiated with light from a light source such as a halogen lamp or a fluorescent lamp and light transmitting through the X-ray film is received by a solid-state image pickup element such as a CCD linear sensor. Then, the X-ray film is scanned to obtain image data.
With respect to a CCD used as the solid-state image pickup element, there is a case where a phenomenon which is called a smear, in which a photoelectric conversion output levels rise by leaking a charge generated in a photoelectric conversion unit or a part of incident light to a transferring unit, occurs. The smear is caused when intense light is entered into the CCD linear sensor. When the smear is caused, the photoelectric conversion output levels of all reading pixels on the transferring unit rise.
For example, in the case where a film in which a low density region is present in a high density region as shown in FIG. 5 is scanned for reading in the illustrated scanning direction, the smear is caused from the light entered into the low density region. Therefore, as shown in FIG. 6, such a problem is caused that levels of right and left regions adjacent to the low density region vary is caused.
Up to now, in order to reduce the smear, a mask is provided on a chip of the solid-state image pickup element to prevent a part of incident light from leaking to the transferring unit. Alternatively, for example, the photoelectric conversion unit may be separated from the transferring unit to prevent a charge generated in the photoelectric conversion unit from leaking to the transferring unit.
Also, there is a method of electrically conducting a smear correction using the output of the solid-state image pickup element. FIG. 7 shows a smear correction circuit for removing the smear, which is provided in an image reading apparatus. The smear correction circuit includes a solid-state image pickup element 1 made from a CCD, an A/D converter 2, a memory 3, a timing generation circuit 4, and a subtracter 5.
The memory 3 temporarily stores a pixel signal A/D-converted by the A/D converter 2. When a record signal is provided, an input signal is written. The written signal is held and outputted. The timing generation circuit 4 controls drive timing of the solid-state image pickup element 1 and write timing to the memory 3. The subtracter 5 subtracts the signal outputted from the memory 3 from the pixel signal outputted from the A/D converter 2.
As shown in FIG. 8, the solid-state image pickup element 1 is composed of photoelectric conversion elements 6, transferring registers 7 that transfer charges generated in the photoelectric conversion elements 6, and an amplifier 8 that amplifies the transferred charges to be converted into voltage values. The solid-state image pickup element 1 has an image taking region that receives light transmitting through a film, an optical black region that shields a part of the photoelectric conversion elements 6, and a dummy region in which the photoelectric conversion elements 6 do not exist.
First, when the light transmitting through the film is imaged onto the solid-state image pickup element 1, charges generated in the photoelectric conversion elements 6 are read to the transferring registers 7 and then transferred in the transferring direction indicated by an arrow. The charges transferred by the transferring registers 7 are amplified by the amplifier 8 and outputted as voltage values in succession. When such operation is repeated while the film is scanned, image signals of the entire film can be obtained.
When intense light is entered into the solid-state image pickup element 1, an output voltage of the solid-state image pickup element 1 increases by a constant level by the influence of the smear. However, because the charges produced from the light entered into the solid-state image pickup element 1 are not present in pixels within the dummy region, only smear components produce. Therefore, the signals from the pixels within the dummy region are subtracted from the signals from the pixels within the image taking region, so that pixel signals from which the smear components are removed can be obtained.
FIG. 9 is a timing chart showing the operation of the smear correction circuit in the image reading apparatus as shown in FIG. 7. As shown in FIG. 9, a pixel signal is outputted from the solid-state image pickup element 1 in synchronization with a horizontal synchronizing signal HD. In addition, as shown in FIG. 9, data is written into the memory 3 for only a period corresponding to the dummy region of the solid-state image pickup element 1. A dummy signal written into the memory 3 is outputted to the subtracter 5 without processing and subtracted from the pixel signal from the image taking region by the subtracter 5. Thus, according to such operation, a pixel signal from which the smear component is removed can be obtained.
According to Japanese Patent Application Laid-Open No. 2000-050165, a method is disclosed, in which an addition average output of a plurality of dummy pixels in which no photoelectric conversion elements are provided or an addition average output of optical black pixels is held in a memory, and a signal (dummy pixel signal or optical black pixel signal) which is held in the memory is subtracted from a pixel signal from a solid-state image pickup element to remove a smear.
However, according to the above-mentioned conventional examples, it is necessary to provide a mask on a chip of the solid-state image pickup element. In the method in which the photoelectric conversion unit is separated from the transferring unit, it is difficult to completely remove the smear. In addition to this, a process for setting the mask is required and the size of the chip increases. Thus, there is a defect that a cost of the solid-state image pickup element increases.
In the method in which the dummy pixel signal or the optical black pixel signal is subtracted from the pixel signal from the solid-state image pickup element, the number of dummy pixel signal and the number of optical black pixel signal are limited to a range of from several pixels to several tens of pixels. Therefore, even if the addition average output is used, there is a problem in that the influences of noises such as a dark current and a random noise, other than the smear, cannot be completely eliminated to cause streak-shaped noises on an image.